1. Field of the Invention
The present invention relates generally to boat structure. More particularly, the present invention relates to boat aft structure which improves low speed performance without adversely affecting high speed performance.
2. Background of the Invention
Propulsion systems for high performance boats commonly employ a setback transom to which an outboard motor is mounted. The transom is "setback" in the sense that the transom is extended rearward of the trailing edges of the underside running surface. Other high performance boats employ a stern drive design where a large motor (often a V-8 car engine or other large engine) is mounted inboard with a propeller outdrive which extends through the transom. While both designs are advantageous once the boat achieves planing speed, low speed performance is adversely affected.
Setback transoms are usually configured in one of two ways. First, a metal bracket (typically in the form of a box which is open at the top and bottom) is attached to the stern of the boat to rearwardly extend the mounting position of the outboard motor. Setting the outboard motor back from the rearward trailing edges of the running surface provides leverage which helps to lift the bow out of the water at high speeds. This reduces the wetted area on the running surfaces with resultant reduction in hydrodynamic drag, thereby allowing the boat to go faster. The amount of leverage desired will dictate the distance the motor is extended rearward of the trailing edge of the boat's running surfaces.
Although the rearward placement of the motor produces desired leverage to lift the bow out of the water during high speed operation, it also causes the stern to sink and the bow to rise during take-off as the boat is trying to come up on plane (also commonly referred to as the "hole shot"). This is undesirable as it increases the boat's hole shot time (i.e., the amount of time required for the boat to achieve planing speed). The setback extension bracket also reduces the stern's buoyancy, which sinks the stern of the boat deeper into the water. This added depth of the motor increases hydrodynamic drag on the lower and mid-section of the motor as well as the setback bracket itself. The motor and the propeller have to overcome this added drag for the boat to come up on plane. In some cases, the boat is unable to achieve planing speed because the increased drag is too great. Thus, while setback motors can dramatically improve the boat's high speed performance, they can be devastating to the boat's ability to plane.
Another method often used to setback an outboard motor is to build a setback transom into the boat. The setback transom effectively creates a step from the trailing edge of the running surface to the surface of the transom itself. While the setback transom provides greater buoyancy at the stern than does a setback bracket, hole shot performance is still hindered as a result of the step. When the boat is sitting at rest, the step under the transom is completely submerged. As the boat begins to move through the water during takeoff, water passes over the step. Since the step is completely submerged and unventilated, the step creates a vacuum which sucks the stern down into the water, thus hindering the boat's ability to come up on plane. The boat cannot plane until this vacuum is released. At the same time, water flows off the step and drags on the lower and mid-section of the motor, which further hinders the boat's ability to reach planing speed.
High performance boats which use a stern drive propulsion system are excessively heavy due to the large, heavy engine placed at the stern with an outdrive through the transom. Although most stern drives are not set back like outboard motors, they are always sluggish out of the hole because the heavy stern has a similar effect of lifting the bow out of the water while sinking the stern into the water. The result is increased drag on the outdrive and aft running surfaces as the boat is coming up on plane. In addition, due to the engine's limited RPM at top speed, larger propellers are needed to keep the engine from over-reving. As a result, low end acceleration is sacrificed.
Efforts to improve the low end performance of high performance boats include the use of fixed and movable trim tabs in a variety of configurations. While such trim tabs assist in planing the boat, they also induce drag and slow the boat's top speed, acceleration, and fuel efficiency.
Thus, there is a need for boat hydro-planing structure which enhances the ability of high performance watercraft to achieve planing speed without adversely affecting the boat's performance once planing speed is achieved.